Fatigue Life Of Specimens Research Articles

Effect of build interruption during laser powder bed fusion process on structural integrity of Ti-6Al-4V

The growing concern for global sustainability highlights the need to address waste generation in metal additive manufacturing (AM) processes. Even with proper planning, unexpected incidents such as software glitches and power outages may interrupt AM processes by pausing the build sequence, leading to a waste of resources (i.e. time, money, and materials). The layer-wise nature of AM processes allows for interrupted builds to be resumed from the stopping point. Nevertheless, interrupting the build process (i.e. pausing and resuming) can compromise the structural integrity of the fabricated parts, potentially leading to localized failure near the interruption location. The objective of this study is to investigate the effect of build interruption during the laser beam powder bed fusion process on the structural integrity of Ti-6Al-4V specimens. Tensile and fatigue tests were performed to assess any possible contribution of build interruption to the specimen failures under monotonic and cyclic loading, and the results were compared with their control counterparts, fabricated without interruption. Tensile and fatigue failures as well as the propagation of a secondary crack were observed at the interruption location. Our experimental observations indicate that when large process-induced defects are present, build interruption plays a peripheral role in the fatigue life and failure mechanism of specimens, with defects being the overriding factor. None of the heat-treated specimens exhibited any tensile or fatigue failures at the interruption location, suggesting that an appropriate heat treatment can be effective in reducing the risk of localized failure near the interrupted location.

Fatigue life evaluation for ring-welded lap joints of stainless steel based on fracture mechanics approach

This study aims to evaluate the fatigue life of ring-welded lap joints of stainless steel with various geometries. It involves welding morphologies observations, static and fatigue tests under tensile-shear loading, and fatigue life evaluation based on fracture mechanics approach. Test results show that the interfacial failure occurs for each specimen under the static loading, and the static strength increases with an increase in weld area, as well as the degree of bending deformation during the test. In addition, three failure modes were identified under fatigue loadings, with crack propagation from the weld root to the weld metal surface accounting for over 90% of the fatigue life for most specimens. Taking into account the fatigue failure modes, a fracture mechanics-based method was used to unify the Fa-Nf curves of ring-welded lap joints with different geometries. This unified curve considers the effects of specimen geometries, crack length, loading ratio, and bending deformation. The analysis demonstrates that the fatigue life of ring-welded lap joints increases with an increase in the thickness of the sheet with a hole and the hole diameter, while decreases with the increasing loading ratio and bending deformation. By utilizing this method, the predicted fatigue life of various ring-welded lap joints can be controlled within a range that is four times of the measured values.

Performance Test of Weld Zone of Rotary Friction Welding of Titanium Alloy Drill Pipe

With the development of complex oil and gas reservoirs, it is difficult for traditional carbon steel to meet the requirements of drill pipe materials in drilling process. Titanium alloy has gradually become a new choice of drill pipe material because of its low density and good corrosion resistance. Titanium alloy has poor weldability and weld zone of titanium alloy drill string is easy to become initial defect and lead to drill pipe failure. Therefore, it is essential to evaluate the performance of titanium alloy drill pipe weld zone. In this paper, the weld zone of titanium alloy rotary friction welding is tested. Metallographic structure, hardness, tension and impact mechanical tests, fracture toughness evaluation and bending fatigue test are carried out. The test results show that the metallographic structure of weld zone changes under the influence of temperature, resulting in different mechanical properties of weld zone from that of pipe body zone. The yield strength and tensile strength in the weld zone are higher than those in the pipe zone, but the fracture toughness is lower than that in the pipe zone. The fatigue life of specimens in weld zone is more than 107 in drilling mud environment. In general, the mechanical properties of weld zone meet drilling requirements.

Effect of Pore Defects on Very High Cycle Fatigue Behavior of TC21 Titanium Alloy Additively Manufactured by Electron Beam Melting

Titanium alloys additively manufactured by electron beam melting (EBM) inevitably obtained some pore defects, which significantly reduced the very high cycle fatigue performance. An ultrasonic fatigue test was carried out on an EBM TC21 titanium alloy with hot isostatic pressing (HIP) and non-HIP treatment, and the effect of pore defects on the very high cycle fatigue (VHCF) behavior were investigated for the EBM TC21 titanium alloy. The results showed that the S-N curve of non-HIP specimens clearly had a tendency to decrease in very high cycle regimes, and HIP treatment significantly improved fatigue properties. Fatigue limits increased from 250 MPa for non-HIP specimens to 430 MPa for HIP ones. Very high cycle fatigue crack mainly initiated from the internal pore for EBM specimens, and a fine granular area (FGA) was observed at the crack initiation site in a very high cycle regime for both non-HIP and HIP specimens. ΔKFGA had a constant trend in the range from 2.7 MPam to 3.5 MPam, corresponding to the threshold stress intensity factor range for stable crack propagation. The effect of pore defects on the very high cycle fatigue limit was investigated based on the Murakami model. Furthermore, a fatigue indicator parameter (FIP) model based on pore defects was established to predict fatigue life for non-HIP and HIP specimens, which agreed with the experimental data.

Prediction of remaining fatigue life of metal specimens using data-driven method based on acoustic emission signal

Aiming at the difficulty of predicting the remaining fatigue life of mechanical parts, a similar fatigue life prediction method based on acoustic emission signals is proposed considering the failure of vibration signals. Moreover, this paper innovatively introduces the temperature signal as a degradation feature to assist the acoustic emission signal feature for fatigue life prediction. The proposed fatigue life prediction method involves multiple processes such as feature extraction, feature smoothing, feature selection, feature compression and health index construction. A variational autoencoder structure combined with a long short-term memory neural network is used to achieve feature compression and retain feature trend. A tensile fatigue test bench was developed to collect the degradation signal from health to fatigue fracture to validate the proposed method. The validation results show that the proposed method can accurately predict the remaining fatigue life. In addition, the role of various data processing methods and the applicability of the proposed method in different working conditions are also discussed.

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

Fatigue damage quantitative evaluation of carbon fiber composites at different stress ratios based on nonlinear ultrasonic

In order to investigate the fatigue behavior of carbon fiber composites and the influence of different stress levels on their fatigue damage characteristics, the fatigue experiments of carbon fiber composites under different stress ratios are carried out. The nonlinear ultrasonic technique is used to online detect the fatigue damage process of carbon fiber composites, the relative nonlinear coefficient β under different stress ratios and different measuring positions is analyzed with the fatigue loading period. The experimental results indicate the relative nonlinear coefficient increases significantly and the rising trend accelerates when the fatigue life of specimen reaches 50%, it is the most sensitive to the change of material microstructure by comparing other parameters and can be used as the characteristic parameter to evaluate the fatigue damage of materials. In addition, the fatigue damage factor D is proposed by analyzing the relationship between the relative nonlinear coefficient and the damage degree of materials, the nonlinear ultrasonic quantitative analysis is carried out and the fatigue damage prediction model based on the relative nonlinear coefficient is established to characterize the severity of fatigue damage of materials. Therefore, the nonlinear ultrasonic quantitative evaluation method for carbon fiber composites is established, it provides a new method and data support for quantitative evaluation of fatigue damage degree of materials by nonlinear ultrasonic method.

A study on dwell-fatigue behavior of additively manufactured Ti-alloy

The Ti6Al4V is a cold dwell sensitive alloy, as evident from the failure investigations of several aerospace fan discs and past laboratory research data. However, the studies on the dwell-fatigue sensitivity of additively manufactured Ti6Al4V alloy and its underlying failure mechanism at room temperature are limited. In this study, the cyclic plasticity behavior and the cold dwell sensitivity of powder-bed fusion based additively manufactured Ti6Al4V alloy has been investigated. The standard test specimens of Ti-alloy were fabricated using the selective laser melting process and were subjected to the low cycle fatigue and low cycle dwell fatigue tests, at the same strain rate. A significant reduction in the fatigue lives of specimens exposed to a dwell period was observed at relatively lower strain amplitudes, while fatigue lives were nearly the same at higher ones. The fractographic analysis of the broken specimens revealed multiple crack initiation regions under the action of dwell loadings. The cracks were initiated either from the process-induced defects or by the mechanism of facet formation in the α laths. The failure mechanism based on stress redistribution and its relaxation are discussed to explain the observed results and are presented in this study.

Fatigue life investigation of notched TC4 specimens subjected to different patterns of laser shock peening

The exhaustion of constructive ways for increasing the service life of parts has led to the development of new methods which can improve their material properties during operation under various loading conditions. Laser shock peening (LSP) induces compressive residual stress field which prevents fatigue crack initiation and propagation in components. Characteristics of laser impact and treatment patterns play an important role in efficiency of LSP application for improvement of fatigue properties. This work is devoted to the experimental examination of two LSP patterns to reveal the most optimal scheme from fatigue live improvement point of view. Proposed LSP pattern allowed one to increase the fatigue life of specimens with semi-circular notch by an order of magnitude. The numerical simulation of the LSP was performed to visualize the residual stress field of treated specimen after loading and to give the interpretation of the experimentally observed improvement of fatigue life.

Experimental investigation on steel bracing members with bolted gusset plate connections

In concentrically braced frames, low-cycle fatigue failure may occur in the brace or bolted connection due to accumulated damage from multiple earthquakes. However, the current research on the hysteretic behavior and fatigue life of H-shaped bracing members with bolted gusset plate connections is lacking, and there is a notable omission regarding the impact of bolt slippage and plastic deformation within the connection. The objective of this study is to address the aforementioned limitation through pseudo-static tests of 8 steel bracing members with bolted gusset plate connections under different clearances, connection coefficients (akin to Ry used in brace design under AISC 341-16 Seismic Provisions), and loading patterns. Results showed that not all loading patterns were suitable for such tests due to bolt slippage. The strength capacity, energy dissipation capacity and deformability of specimens with five types of clearances displayed small variation in tests. It has been demonstrated that utilizing a connection coefficient (1.03) lower than the prescribed value (1.3) can still result in a favorable hysteretic performance. The effective length factor of the bracing member under double bolted gusset plate connections is estimated to be 0.79, which can provide valuable information for optimizing the brace design. Additionally, a method for predicting the fatigue life of test specimens is proposed. The fatigue performance and failure mode of the bracing members under bolted connections with clearances of 0t, -2t, -4t and -6t (where t represents the thickness of the gusset plate) are superior to those under welded connections.

Analysis of Chinese ancient brick masonry characteristics under uniaxial compression fatigue loading

Urban expansion and diversified traffic aggravate the damage and deterioration of ancient buildings. It is crucial to evaluate the long-term fatigue performance of ancient brick masonry structures for the safety and maintenance of ancient buildings. A series of indoor tests were conducted on the ancient brick masonry. The images for the whole process of the test were collected using VIC-3D technology. The relationship between the stress level and the number of failure cycles under compressive fatigue load and the stress–strain evolution under different stress levels were determined. A mathematical model of p-S-N fatigue life prediction based on probability was proposed, which comprehensively considered the maximum stress level, stress range, number of load cycles, and survival probability. The equivalent fatigue life of ancient brick masonry specimens under each failure probability obeys Weibull distribution, and then the fatigue life of masonry can be predicted at any expected confidence level. The test results provide reliable theoretical and data support for fatigue deterioration and life prediction of ancient masonry structures in service.

Random vibration fatigue behavior of directionally solidified superalloy: Experiments and evaluation of life prediction methods

Vibration is closely related to structural dynamics, and the study of random vibration fatigue loads is typically conducted in the frequency domain. In this study, various frequency domain methods were employed to predict the random vibration fatigue life of DZ125L directionally solidified superalloy under different vibration signal intensities. Experimental analysis was used to examine the fatigue life and random vibration response of DZ125L alloy specimens under different vibration signal intensities. The S-N curve derived from sinusoidal vibration tests was used to replace the traditional S-N curve obtained from tension-compression tests in random vibration fatigue analysis, leading to improved life prediction accuracy. Additionally, an intensity function fξ was established using the spectral width parameters α1 and α2 of the random process and the vibration signal intensity ξ, which was used to modify the traditional Rayleigh method. A new vibration fatigue life prediction model based on frequency domain method is obtained. The results indicate that the intensity function fξ emphasizes the elastic or plastic stage of the specimen during the random process, leading to highly accurate life prediction values using the Modified Rayleigh method across different vibration signal intensities, with all prediction points are within 1.5 times the scatter band.

Efficient optimization framework for L-PBF fatigue enhanced Ti6Al4V lattice component

Industries today face challenges in incorporating metallic additively manufactured lattice structures in critical components subjected to fatigue loading. This work explores the relationship between fatigue properties and the printing orientation of Laser-Powered Bed Fusion (LPBF) lattice structures. This relation is at the base of a cost-effective and time-efficient optimization workflow able to determine the optimal lattice printing orientation for improved fatigue life. Fatigue resistance is tested under uniaxial conditions on miniaturized specimens that mimic the lattice sub-unit elements: struts and nodes. The collected data is used as input for the optimization algorithm to determine the specimen orientation that maximizes fatigue life. The optimized specimens are manufactured, tested under three-point-bending conditions, and analysed using metrological x-ray computed tomography to verify the improvement. The proposed workflow is able to produce a 24 % increase in specimen fatigue life by simply adjusting the orientation on the printing plane.

Experimental and numerical investigation of thermal fatigue life in wedge specimens made of Ni-resist D5S cast iron

Thermal fatigue experiments were performed on a single-edge wedge specimen of Ni-resist D5S cast iron to reproduce the conditions to which exhaust manifolds are subjected during service. The leading edge temperature was cycled between 20°C and 850°C, and the temperature distribution on the specimen surface was measured with thermocouples during thermal cycling. Due to the complexity of the loads, a uncoupled thermomechanical computation of the problem was performed using the three-dimensional finite element code ABAQUS to evaluate the temperature, stress and strain distribution over a thermal fatigue specimen. The heat fluxes and forced convection coefficients generated by the thermal fatigue benchmark were optimized in both heating and cooling phases by reverse engineering to reproduce the experimental temperature-time profiles measured by thermocouples on the surface of the specimen. Then, an elastic-viscoplastic tow-layer model was used to simulate the mechanical behavior of the studied material. It was found that the maximum stress value was located in the center of the specimen at the edge of the wedge. This result was confirmed by experimental observations. The crack initiation zone is located in the edge zone and the direction of crack propagation is perpendicular to the edge line toward the thick part of the specimen. The saturation of the main crack propagation near the uniform specimen zone was mainly explained by the rapid decrease of the energy dissipated per cycle.

Green electrochemical polishing of EBM Ti6Al4V samples with preliminary fatigue results

Additive manufacturing technologies have several well-known advantages; however, the surface quality of the printed parts is poor, compromising the corrosion and mechanical properties. Thus, surface finishing treatments are necessary. The most post-processing treatments proposed in the literature are mechanical or chemical. The formers are contact techniques and cannot be suitable for complex geometry, while the latter use strong and dangerous reagents. This paper is aimed to investigate the effect of the electropolishing treatment, a contactless technique, using an eco-friendly electrolytic solution in reducing the roughness and improving the fatigue properties of Ti6Al4V specimens produced by electron beam melting technology. Several parameters affecting the effectiveness of the electropolishing treatment have been investigated, as interelectrode distance, stirring rate and duration time. An optimal combination of them allowed a uniform surface treatment and a significant roughness reduction (64% of Sa reduction for cylindrical specimens). Under sinusoidal loading conditions with a maximum stress of 378 MPa and a stress ratio equal to R = 0.1, the mean fatigue life of electropolished dog-bone specimens was 250% longer than that of as-built specimens.

Effects of high-energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness, and fatigue life and strength of single-crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser-peened single-crystal nickel superalloy specimens compared to unpeened and shot-peened specimens following hot corrosion exposure and then fatigue testing. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. The latter treatment supports the benefit of a unique process involving application of layers of laser peening using high energy with large footprint spots combined with interspersed cyclic annealing. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Unpeened and peened specimens were exposed to sulphate corrosives at 700 °C for 300 h and then fatigue tested. Tests of five non-laser-peened specimens all failed in low-cycle fatigue regime, whereas three identically tested laser-peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests also showed fatigue strength improvement of 2:1 by laser peening. Residual stress measurements post hot-corrosion exposure and fatigue testing showed notable 5 mm depth retention of residual eigenstress in a laser-peened specimen.

Understanding geometrical size effect on fatigue life of A588 steel using a machine learning approach

In this paper, both experimental and machine learning results show that the fatigue life of A588 steel specimens with different gauge lengths and widths varies more greatly compared with that of the specimens with different thicknesses as the gauge dimensions are reduced from 15 mm to 1.5 mm. The optimal machine learning algorithm is derived to predict the fatigue life of specimens with a thickness of 1 mm, and the predicted results are verified by the fatigue experiments.

Improvement of titanium alloy TA19 fatigue life by submerged abrasive waterjet peening: Correlation of its process parameters with surface integrity and fatigue performance

Submerged abrasive waterjet peening (SAWJP) is an effective anti-fatigue manufacturing technology that is widely used to strengthen aeroengine components. This study investigated the correlation of SAWJP process parameters on surface integrity and fatigue life of titanium alloy TA19. SAWJP with different water pressures and standoff distances (SoDs) was conducted on the TA19 specimens. The surface integrity of the specimens before and after SAWJP with different process parameters was experimentally studied, including microstructure, surface roughness, microhardness, and compressive residual stress (CRS). Finally, fatigue tests of the specimens before and after SAWJP treatment with different process parameters were carried out at room temperature. The results highlighted that the fatigue life of the TA19 specimen can be increased by 5.46, 5.98, and 6.28 times under relatively optimal process parameters, which is mainly due to the improved surface integrity of the specimen after SAWJP treatment. However, the fatigue life of specimens treated with improper process parameters is decreased by 0.55 to 0.69 times owing to the terrible surface roughness caused by the material erosion. This work verifies that SAWJP can effectively improve the surface integrity and fatigue life of workpieces, and reveals the relationship between process parameters, surface integrity, and fatigue life, which provides support for the promotion of SAWJP in the manufacturing fields.

Cooling rate effects on fatigue life prediction using hardness measurements for in-service steel patch-welds with and without TIG dressing treatment

In-service patch welding is commonly used to repair corroded or damaged steel infrastructures. However, the in-service conditions may unfavorably affect the long-term mechanical and structural integrity. The present research studies the fatigue behavior of fillet welded patch repairs with and without TIG-dressing (TD). Six types of panels were constructed under various in-service cooling conditions. Specimens cut from the main panels were subjected to high-cycle fatigue, hardness, and metallography tests. The fatigue life of the specimens was also predicted using a novel numerical approach. The model incorporated micro-hardness test data from the weldment section. Macrographic and microhardness data were used for the zoning of the weldment section. The harness data were then employed to define the post-weld mechanical properties of the materials in different weldment zones. The fatigue life was estimated using the microhardness data, stress and strain data obtained from the finite element analysis of the specimen, a UVARM subroutine, and several multiaxial fatigue life prediction models. The numerical fatigue life estimations showed reasonable agreement with the experimental data. Both experimental and numerical modeling results proved significant improvements in the fatigue life of the TD specimens. Regardless of the welding conditions/treatments, the best fatigue life estimates were provided by the BMM, SWT, Glinka, and KBM criteria, respectively.

Effect of bending-torsion on fracture and fatigue life for 18Ni300 steel specimens produced by SLM

In this study, different fracture surfaces caused by fatigue failure were generated from 18Ni300 steel produced by selective laser melting (SLM). Hollow round bars with a transverse hole were tested under bending-torsion to investigate the crack initiation mechanisms and fatigue life. Next, the post-failure fracture surfaces were examined by optical profilometer and scanning electron microscope. The focus is placed on the relationship between the fatigue features (e.g., bending-torsion ratio, fatigue crack initiation angles, and fatigue life) and the fracture surface topography parameters (e.g., height parameter Sx, volume parameters Vx, maximal pit and valley angles). The analysis was carried out using the entire fracture surface of the tested specimens. It was found that the decrease of the shear stress level significantly reduces the value of the fracture surface parameters. A fatigue life prediction model based on both the surface topography values and the applied load was proposed. Fatigue life predictions for different loading ratios agreed well with the experimental results and were slightly better than those of other existing models. The proposed model can be helpful for post-mortem analysis of engineering components subjected to multiaxial fatigue.